It is often required to couple data from a first or primary circuit to a second or secondary circuit in a manner to maintain an electrical isolation between the primary and secondary circuits. Typically, this is carried out using either an opto-coupler or a transformer.
An example of such a data coupling requirement is in providing, to a control unit on the secondary or output side of an isolating switch-mode power supply (SMPS), data relating to monitoring of an input voltage and/or current supplied from a power source to the primary side of the SMPS. For example, the power source may be a 48 volt battery supply of a telephone central office.
Especially in such an example, in which a low operating voltage for the primary side monitoring and data transmission circuits may be derived from the 48 volt supply using a voltage-dropping resistor or a linear voltage regulator, it is desirable for the data transmission circuit to operate at a low current. A low cost and small size for the data coupling components are also desirable. Opto-couplers can have a relatively high current requirement and a relatively high cost, and accordingly transformer data coupling may be preferred.
It is therefore desirable to provide a low cost and low power transformer data coupler.
Common mode immunity of such a data coupler is also an important consideration. For example, it is desirable for the data coupler to accommodate electrical fast transients (EFTs) having a rise time of the order of 100 kV/μs (much more than the order of 10 kV/μs for the best opto-couplers) with a peak of the order of 4 kV. Accordingly, it is necessary for a data coupler to have a high common mode immunity, so that such EFTs, which can be coupled to the secondary circuit via the inter-winding capacitance of the transformer, do not cause damage to the data receiver or other parts of the data coupler. Although such common mode transients may result in bit errors in the data coupled via the data coupler, recovery from such errors is relatively easily provided using well-known data communication techniques.
There is therefore a need to provide a transformer data coupler having high common mode immunity, low cost, low power requirements, and small size.